1. Field of the Invention
The present invention relates to an apparatus and a method for non-stop switching of a signal from a current system to a spare system under a PCM transmission system. More particularly, this invention is concerned with an apparatus and a method for non-stop switching in which the digital network is duplicated to ensure instantaneous switching when a fault occurs.
A method called asynchronous transfer mode (hereafter, ATM) has been proposed as a technological element for implementing a broad-band ISDN in recent years. The ATM is a method of segmenting user information in fixed-length transfer units referred to as cells, of about 50 bytes, then transmitting the cells to the destinations in a network that are specified in the address headers of the cells at high speeds. According to the ATM, and unlike conventional packet switching, information need not be temporarily placed in a memory but can be transferred at high speeds in cell units merely by interpreting the cell headers. A flow control, error recovery control, and other protocols are not implemented, permitting the high-speed transfer of cells.
The ATM, in which cells are allocated depending on the presence or absence of user information, permits communication at any transmission speed and facilitates efficiency in information transmission despite burst traffic. A unified information unit or cell is employed, helping thereby to achieve multimedia communication of diversely-formatted information effortlessly and economically.
2. Description of the Related Art
In a digital network, since signals are usually highly multiplexed, a fault in a signal affects a wide region of the network. Therefore, a transmission system or switching apparatus is designed to have a redundant configuration to ensure system reliability. The redundant configuration is made up of one spare unit or line for m current units or transmission lines. When a fault occurs in a unit or line, the unit or line is replaced with a spare. However, in a system for 10 providing complex and quick services over a digital network, a duplex standby configuration is usually adopted because of the ease by which it can be controlled.
In the duplex standby configuration, a spare system is placed in hot-standby state and operated in the same manner as a current system at all times. If a fault occurs in the current system, the data of both systems is compared in bits. This facilitates efficiency in fault detection. In the case of an occurrence of a fault, the current system is switched to the spare system instantaneously. This helps improve the reliability of a transmission system. In conventional transmission services since a continuous-bitstream oriented (CBO) service is provided, an information stream flows at a constant speed without fluctuation. In packet switching or synchronous transfer mode (STM), store-and-forward switching is performed using processors at end offices. Therefore, phase control and other supports can be provided at the end offices. Even if a phase lag is existent between information transmitted from the spare system and that from the current system, switching does not cause deterioration in information quality, such as duplication of signals or missing signals.
However, in an ATM transmission system, for example, user information is transferred in fixed-length blocks or cells, which are dynamically allocated in response to time-dependent, varying users' requests for information transmission. Therefore, even if quick switching is performed in a selector device, part of a cell-segmented signal is likely to be missing. In addition, since intervention of protocols in a transmission network is minimized to ensure the high-speed transfer of cells, a phase difference between spare and current systems cannot be easily removed. Not only does a missing cell-segmented signal occur but, also, a signal may be duplicated and transferred if the switching is done according to incorrect timing. This may change the contents of other cell data and cause a variety of unfavorable effects.